G3 G3 - Gas Technician Level 3 Appliance Installation and Service Questions and Answers 3 — Questions and Answers
Question 1: A gas furnace is short-cycling (turning on and off rapidly). The most likely cause is:
- Incorrect gas pressure at the meter
- Overheating due to restricted airflow causing the high-limit switch to trip (Correct answer)
- A faulty gas valve
- Incorrect flue pipe diameter
Correct answer: Overheating due to restricted airflow causing the high-limit switch to trip
Restricted airflow causes the heat exchanger to overheat, tripping the high-limit safety switch and shutting off the burner prematurely.
Question 2: When a gas appliance produces a strong smell of sulfur (rotten eggs) after lighting, the technician should:
- Assume it is normal and continue operation
- Immediately shut off the gas supply and investigate for a gas leak (Correct answer)
- Increase the burner firing rate
- Clean the burner orifices
Correct answer: Immediately shut off the gas supply and investigate for a gas leak
Sulfur odor indicates the odorant added to natural gas, signaling a possible gas leak that requires immediate shutdown and investigation.
Question 3: If a gas boiler pressure relief valve (PRV) is dripping frequently, the technician should first check:
- The burner flame color
- System pressure and the expansion tank charge for correct pre-charge pressure (Correct answer)
- The flue temperature
- The gas meter output
Correct answer: System pressure and the expansion tank charge for correct pre-charge pressure
A PRV that opens repeatedly usually indicates the system pressure exceeds its set point, often due to a waterlogged or improperly charged expansion tank.
Question 4: A gas appliance vent pipe is found to be incorrectly sloped downward toward the appliance. This is a problem because:
- It increases draft velocity
- Condensate and flue gases will not flow toward the exterior, risking backdrafting and CO spillage (Correct answer)
- It makes the vent pipe look uneven
- It reduces gas pressure at the burner
Correct answer: Condensate and flue gases will not flow toward the exterior, risking backdrafting and CO spillage
Vent pipes must slope upward toward the termination to ensure condensate drains away and flue gases rise and exit the building.
Question 5: During a combustion analysis on a gas furnace, the technician measures an O2 reading of 15%. This indicates:
- Good combustion efficiency
- Excessive dilution air (too much excess air), reducing combustion efficiency (Correct answer)
- Carbon monoxide is present
- The heat exchanger is cracked
Correct answer: Excessive dilution air (too much excess air), reducing combustion efficiency
An O2 reading of 15% indicates far too much excess air is entering the flue, diluting combustion gases and wasting heat energy.
Question 6: A homeowner complains that their gas fireplace pilot stays lit but the main burner won't come on when the thermostat calls for heat. The most likely cause is:
- The pilot orifice is too large
- A faulty thermopile not generating enough millivoltage to open the main gas valve (Correct answer)
- Incorrect flue size
- The gas pressure is too high
Correct answer: A faulty thermopile not generating enough millivoltage to open the main gas valve
A thermopile generates the millivoltage needed to energize the main valve; if output is below specification, the valve won't open even with a healthy pilot.
G3 G3 - Gas Technician Level 3 Appliance Installation and Service Questions and Answers 3
A gas furnace is short-cycling (turning on and off rapidly).
The most likely cause is: